Multilayer metal composite products obtained by compound strand casting

ABSTRACT

A metallurgical product, such as brazing sheet, comprises a core, on at least one side of the core an interlayer bonded to the core and a cladding bonded to the interlayer. The core and the interlayer are a compositely cast material having at their mutual interface a bond formed in a casting process by their simultaneous solidification from contacting melts of their respective materials. This provides a simple process, with good adhesion. The cladding is applied subsequently and the composite material is rolled into a sheet or plate product.

FIELD OF THE INVENTION

The invention relates to a metallurgical product comprising a core, onat least one side of the core an interlayer bonded to the core and acladding bonded to the interlayer, and to a method of producing such ametallurgical product. Particularly but not exclusively, the inventionrelates to such a metallurgical product in the form of a brazing sheetor a brazing plate, and also to a brazed assembly including themetallurgical product as at least one component.

DESCRIPTION OF THE PRIOR ART

A metallurgical product comprising a core alloy with on at least oneside an interlayer and at least one cladding layer, which is bonded tothe interlayer has been obtained by roll cladding the different layerstogether. U.S. Pat. No. 2,821,014 typically describes this method andproduct. A disadvantage of roll cladding is the handling and processing,e.g. rolling and surface treatment, of all the different types oflayers. Consequently roll cladding is time consuming and is expensive.

Direct adhesion of clad layers to a core layer to make brazing sheet byrolling is also known, e.g. from EP-A-326337.

U.S. Pat. No. 5,476,725 describes a method of manufacture of a compositebrazing product having an aluminium core and at least one clad layer.The clad layer or layers in band form are fed to opposed cooled rollssimultaneously with molten core material, which is solidified in contactwith the clad layer or layers. It is stated that, despite the fact thatthe clad layer melts at a lower temperature than the core, it does notmelt when it contacts the molten core material in the roll bite.Subsequently the composite is cold-rolled. In one variant of the method,interlayers of zinc are thermally sprayed onto the clad bands as theyare unwound from coils prior to the contact of the bands with the moltencore material. A similar process is described in JP-A-56-091970.

JP-A-61-111745 describes a method of making a material intended forbrazing to another member to provide a corrosion and wear resistantlayer. Brazing metal is solidified as a band on one roll of a pair ofrolls, and a second molten metal in the form of a Co alloy containingceramic fibres is solidified onto the band at the nip of the two rolls.

Some further prior art relating generally to casting processes isdiscussed below.

SUMMARY OF THE INVENTION

An object of the invention is to provide a metallurgical product of thetype described above which can be produced at lower costs compared tothe existing products.

According to the invention in a first aspect there is provided ametallurgical product comprising a core, on at least one side of thecore an interlayer bonded to the core and a cladding bonded to theinterlayer, characterized in that said core and interlayer are acompositely cast material having at their mutual interface a bond formedby their simultaneous solidification from contacting melts of theirrespective materials in a casting process.

According to the invention in a second aspect there is provided ametallurgical product comprising a core, on at least one side of thecore an interlayer bonded to the core and a cladding bonded to theinterlayer, characterized in that said core and interlayer are acompositely cast material having at their mutual interface a bondingzone formed by mutual mixing of their respective compositions in themolten state prior to their solidification in a casting process.

The metallurgical product in accordance with this invention is thereforecharacterized in that the core and the interlayer are bonded in acasting process in which the core and the interlayer are solidifiedsimultaneously from the molten state as to form a composite material.Subsequently, the clad layer is applied. The core alloy and theinterlayer alloy are simultaneously solidified from the molten stateduring a continuous or semicontinuous casting process in such a mannerthat the obtained solidified core is covered at least on one side, andpreferably on the whole outer surface, with the interlayer alloy so asto form a compositely cast material. Such a compositely cast material isobtained at significant lower costs than a conventional roll cladcomposite material, because scalping of the core alloy and the steps ofcasting, rolling and surface treatment of the interlayer prior tobonding onto the core are no longer required.

A further advantage of the invention is the excellent strong bonding ofthe interlayer and the core, due to their simultaneous solidification incontact with each other. This contact of the two molten materials withconsequent mixing at their interface produces in the solidified producta narrow interface bonding zone which has a metallurgical naturecharacteristic of the product and method of this invention. Such aninterface bonding zone is readily recognizable by the expert in theproduct, and is different from and distinguishable from the prior artproducts described above in which one or both of two layers is solidprior to the bonding.

In the metallurgical product in accordance with the invention, when inplate or sheet form, at least on one side of the core the interlayerthickness is preferably 1-30% of the thickness of the compositely castmaterial. In the case of a product of the invention in the form of abar, rod or wire, the interlayer is likewise preferably 1-30% of theradius (half width) of the compositely cast material. This allows for agreat flexibility in thickness of the interlayer, this flexibility beingsignificantly larger compared to roll cladding.

It is an advantage obtainable with the invention that the core and theinterlayer are essentially free from oxides at their interface. Animportant advantage of such a compositely cast material compared to rollbonded material is the improved bonding between the core and theinterlayer due to the absence of oxides or oxide-compounds at theirinterface. Such compounds can be present in roll bonded material evenafter proper surface treatment prior to roll bonding.

In the metallurgical product in accordance with the invention thecladding may have a solidus temperature different from that of the coreand more preferably the cladding has a solidus temperature lower thanthat of the core. The metallurgical product in these cases embodimentsis very suitable for brazing sheet applications.

While the invention is in principle applicable to a wide range ofmetals, preferably the core is an aluminium alloy, which makes theproduct suitable for a wide range of applications, e.g. brazing sheet.Depending on the required final properties for the application or use ofthe metallurgical product, one may select a heat treatable aluminiumalloy as the core alloy, e.g. AA2xxx, AA6xxx or AA7xxx-series alloys, ora non-heat treatable aluminium alloy, e.g. AA3xxx or AA5xxx-seriesalloys.

When the cladding is a brazing alloy, preferably it is an aluminiumbrazing alloy. Depending on the final use or application of themetallurgical product of the invention, also another cladding can beapplied, e.g. AA1xxx-series alloys to obtain a mirror like surfacefinish, or the application of zinc or a zinc alloy or a zinc containingaluminium alloy to improve corrosion resistance.

Preferably the interlayer has a corrosion potential different from thatof the core, and more preferably lower than that of the core. In thiscase the interlayer acts as an anodic interlayer and improves thecorrosion resistance of the metallurgical product. Depending on the typeof core alloy and the application or use of the metallurgical productone may prefer an interlayer comprising a heat treatable aluminiumalloy, e.g. AA2xxx, AA6xxx or AA7xxx-series alloys, or an interlayercomprising non-heat treatable aluminium alloys, e.g. AA1xxx, AA3xxx orAA5xxx-series alloys. Alternatively the use of zinc or a zinc alloy or azinc-containing aluminium alloy is possible.

Preferably the interlayer constitutes, in known manner, a diffusionbarrier between the cladding and the core. In this case the interlayershields the core material from elements coming from the molten brazingalloy during brazing and vice versa. Depending on the type of core alloyand the application or use of the metallurgical product, one may preferan interlayer AA6xxx or AA7xxx-series alloys, or one may prefer aninterlayer comprising a non-heat treatable aluminium alloy, e.g. AA1xxx,AA3xxx or AA5xxx-series alloys. See U.S. Pat. No. 2,821,014 for furtherinformation concerning barrier liners for use in the present invention.

Another object of this invention is to provide a method for producingthe metallurgical product of the invention.

The method in accordance with this invention comprises the steps of

(i) preparing the compositely cast material comprising the core and theinterlayer bonded to the core by a process including casting the coreand the interlayer in such a manner that their respective melts solidifysimultaneously in contact with each other, and

(ii) bonding the cladding to said interlayer after solidification ofsaid interlayer.

An advantage of this method is that the core and the interlayer arebonded together in one process step, avoiding the need for rolling aplate of the interlayer material and surface treatments of the core andthe interlayer material. In this way significant process cost benefitsare obtained. Another advantage of the method according to the inventionis the flexibility of choice of the core alloy. Non-conventional corealloys can be processed, e.g. high strength AA7xxx-series alloys havinga functional interlayer between the cladding and the core.

Any suitable casting process, particularly continuous andsemi-continuous casting processes can be used in the method of theinvention to form the compositely cast materials, e.g. the castingtechniques or processes known from the patent publications EP-A-0596134and DE-A-4420697. It is mentioned here for completeness of prior artdisclosure that many other combined casting techniques in which twometals are solidified are known. Examples are those of DE-A-844806, U.S.Pat. No. 4,567,936, GB-A-2204518 and EP-A-596134, but these do notinvolve simultaneous solidification of two molten metals in contact.

More preferably a DC-casting process or modification thereof is used,e.g. that of DE-A-4420697, because in this way the method can be appliedat existing casting stations without the need for significant costinvestments for a new casting machine and required infrastructures, bothfor ingots and extrusion billets.

In a preferred method of the invention the thickness of the compositelycast material after casting is more than 100 mm, and in the case of anextrusion billet the diameter of the composite material is more than 100mm, although smaller diameter billets can be produced in the way(“extrusion billet” is a term used in the art to describe typically acylindrical product which may be subjected to extrusion, though in thepresent invention a different subsequent treatment such as rolling maybe employed). This allows for the production of the composite materialusing conventional DC-casting techniques or small modifications thereof.Further cost reductions are achieved in this way.

The bonding between the cladding and the interlayer can be obtainedusing the conventional roll cladding technique or modifications thereof.However, also other techniques like dipping, thermal spraying or arcspraying the cladding onto the interlayer may be used.

Background information on plating, coating, and surface treatment may befound in the following two resources and in the references cited intheir bibliographies:

(1) Metals Handbook, 9th edition, Vol. 5, “Surface Cleaning, Finishingand Coating”, American Society for Metals (ASM), Metals Park, Ohio(1982); and

(2) ASM Handbook, Vol. 18, “Friction, Lubrication and Wear Technology”,ASM International, Metals Park, Ohio (1992).

Among the various options of plating, coating and surface treatment,roll cladding is preferred for plate and/or sheet products of thepresent invention.

In the method of the invention it is preferred that the compositely castmaterial is homogenized, and preferably homogenized prior to step (ii).Depending on the type of alloy chosen for the core and the interlayer,homogenization may be required to enhance processing of the compositematerial for instance to improve formability during rolling.Homogenization is a known process step performed to reduce residualstresses from casting and reduce micro-segregation of alloying elements.

Preferably the compositely cast material is hot rolled prior to step(ii). Depending on the thickness required of the compositely castmaterial it may be rolled, and more preferably hot rolled, prior to thecladding. Preferably the compositely cast material is homogenized priorto hot rolling.

The compositely cast material is typically surface treated prior to step(ii). By surface treating, e.g. scalping, chemical etching and manyother possible treatments or combinations thereof, the bonding betweenthe interlayer and the cladding may be improved.

The method may further include a step (iii) of rolling the product intoa sheet product or plate product. Heat treatments at any stage during orafter rolling may also be applied to obtain the required finalproperties of the plate or sheet product or may be applied to enhanceprocessing during rolling.

The invention further relates to a sheet product obtained fromprocessing the metallurgical product according to the invention orobtained from the method according to the invention, wherein the sheetproduct has a thickness of less than 3 mm and more preferably less than2 mm. More in particular the invention also relates to a sheet productobtained from processing the metallurgical product according to theinvention or obtained from the method according to the invention,wherein the sheet product is brazing sheet. Using the metallurgicalproduct according to the invention a wide variety of possiblecombinations of core, interlayer and cladding can be obtained which arevery suitable for brazing sheet and brazing sheet applications. Forexample the cladding comprises Si in a range of 3-18 weight %, orcomprises an aluminium alloy originating from the AA4xxx-series alloys.

Further the invention relates to a plate product obtained fromprocessing the metallurgical product according to the invention orobtained from the method according to the invention, wherein the plateproduct has a thickness in a range of 3-8 mm.

The invention also relates to the use of the metallurgical productaccording to the invention or obtained from the method according to theinvention as brazing sheet, e.g. as part of an assembly and bonding theassembly together. Or for use as part of an assembly and bonding theassembly together by heating the assembly above the solidus temperatureof the cladding. Or as part of an assembly comprising a heat exchangerof tube and fin or plate and fin construction kept in heat exchangingrelationship by mechanical connection. Or for use as part of an assemblycomprising a heat exchanger of tube and fin or plate and finconstruction kept in yet another example for suitable use is as part ofan assembly comprising a heat exchanger or tube and fin or plate and finconstruction kept in heat exchanging relationship by the moltencladding.

EXAMPLE

On a pilot scale in a test the following multi-layer product wasproduced.

A billet with a length of about 1500 mm and a total diameter of 255 mmwas cast using the casting technique of DE-A-4420697, with simultaneoussolidification of core and interlayer while their respective melts arein contact. The diameter of the core was 221 mm and the remainder wasthe interlayer. The core alloy has a chemical composition within theAA6063 range and the interlayer was within AA3003 range with a furtheraddition of 1.3 wt. % zinc. The cast composite billet was cut lengthwiseinto sections of 500 mm. The sections were homogenized for 8-12 hours at560-580° C. with a heat-up and cooling rate of about 35° C./hour. Thesections were hot-rolled into a rectangular shape of required thickness.Alternatively forging might be applied. The interlayer was scalped inorder to obtain the required thickness of the layer in the finalthickness. The scalped section was roll clad with a cladding on theinterlayer on one side only. The cladding material was within the AA4004range. The multilayer product was hot rolled down to 3.5 mm and furthercold rolled to 0.35 mm. During cold rolling an interanneal was appliedat a temperature of 360° C. The final multilayer sheet product has athickness of 0.35 mm of which about 10% is the clad layer and 35-45 μmis the interlayer. The multilayer brazing sheet product was tested in aSWAAT-test in accordance with ASTM G-85 and had a corrosion life of morethan 25 days without perforations. This is a similar test resultcompared with multilayer brazing sheet obtained by conventional rollcladding of similar composition and thickness of the different layers.

In this manner a multilayer product is obtained that has the followingadvantages:

has corrosion and mechanical properties similar to that of conventionalroll clad material;

has a great flexibility in alloy composition;

can be produced at existing production facilities;

is produced at lower cost;

has an improved bonding between the core and the interface layer.

What is claimed is:
 1. A metallurgical product comprising a core, on atleast one side of the core an interlayer bonded to the core and acladding bonded to the interlayer, wherein said core and interlayer area compositely cast material having at their mutual interface a bondingzone formed by mutual mixing of their respective compositions in themolten state prior to their solidification in a casting process.
 2. Ametallurgical product according to claim 1, which is plate or sheet andin which, on at least one side of said core, said interlayer has, in thethickness direction of said plate or sheet, a thickness in the range 1to 30% of the thickness of said compositely cast material.
 3. Ametallurgical product according to claim 1, wherein said the shape ofsaid product is selected from the group consisting of a bar, a rod and awire, and in the section transverse to the length direction of saidshape said interlayer has a thickness which is in the range 1 to 30% ofthe radius of said compositely cast material.
 4. A metallurgical productaccording to claim 1, wherein the core and the interlayer areessentially free from oxides at their interface.
 5. A metallurgicalproduct according to claim 1, wherein the cladding has a solidustemperature different from that of the core.
 6. A metallurgical productaccording to claim 5, wherein the cladding has a solidus temperaturelower than that of the core.
 7. A metallurgical product according toclaim 5, wherein the cladding is a brazing alloy.
 8. A metallurgicalproduct according to claim 7, wherein the cladding is an aluminiumbrazing alloy.
 9. A metallurgical product according to claim 1, whereinthe core is an aluminium alloy.
 10. A metallurgical product according toclaim 9, wherein the core comprises a heat treatable aluminium alloy.11. A metallurgical product according to claim 10, wherein the corecomprises a non-heat treatable aluminium alloy.
 12. A metallurgicalproduct according to claim 1, wherein the interlayer has a corrosionpotential different from that of the core.
 13. A metallurgical productaccording to claim 1, wherein the interlayer has a corrosion potentiallower than that of the core.
 14. A metallurgical product according toclaim 1, wherein the interlayer acts as a diffusion barrier between thecladding and the core.
 15. A metallurgical product according to claim12, wherein the interlayer comprises a heat treatable aluminium alloy.16. A metallurgical product according to claim 12, wherein theinterlayer comprises a non-heat treatable aluminium alloy.
 17. Ametallurgical product according to claim 1, in the form of sheet orplate.
 18. A method of producing a metallurgical product according toclaim 1, comprising the steps of (i) preparing said compositely castmaterial comprising said core and said interlayer by a process includingcasting said core and said interlayer in such a manner that theirrespective melts solidify simultaneously and in contact with each other,and (ii) bonding said cladding to said interlayer after solidificationof said interlayer.
 19. A method according to claim 18, wherein, aftercasting, said compositely cast material has a thickness of at least 100mm.
 20. A method according to claim 18, wherein said compositely castmaterial is subjected to a homogenization heat treatment prior to step(ii).
 21. A method according to claim 18, wherein said compositely castmaterial is hot rolled prior to step (ii).
 22. A method according toclaim 18, wherein said compositely cast material is subjected to asurface treatment prior to step (ii).
 23. A method according to claim18, wherein after step (ii), the product is rolled into a sheet or plateproduct.
 24. A metallurgical product according to claim 1 in the form ofa sheet product of a thickness less than 3 mm.
 25. A metallurgicalproduct according to claim 1, in the form of brazing sheet.
 26. Ametallurgical product according to claim 25, wherein said claddingcomprises an aluminium alloy having a Si content in the range 3 to 18weight %.
 27. A metallurgical product according to claim 1 in the formof a plate product having a thickness in the range 3 to 8 mm.
 28. Abrazed assembly comprising at least one component which is ametallurgical product according to claim
 1. 29. A method of producing ametallurgical product according to claim 3, comprising the steps of (i)preparing said compositely cast material comprising said core and saidinterlayer bonded to said core by a process including casting said coreand said interlayer in such a manner that their respective meltssolidify simultaneously and in contact with each other, and (ii) bondingsaid cladding to said interlayer after solidification of saidinterlayer.
 30. A method of producing a metallurgical product accordingto claim 18, comprising the steps of (i) preparing said compositely castmaterial comprising said core and said interlayer bonded to said core bya process including casting said core and said interlayer in such amanner that their respective melts solidify simultaneously and incontact with each other, and (ii) bonding said cladding to saidinterlayer after solidification of said interlayer.
 31. A brazedassembly comprising at least one component which is a metallurgicalproduct according to claim
 1. 32. A brazed assembly comprising at leastone component which is a metallurgical product according to claim 17.33. A metallurgical product according to claim 1, wherein said core andsaid interlayer comprise a continuous or semi-continuous cast product.34. A metallurgical product according to claim 17, wherein said core andsaid interlayer comprise a continuous or semi-continuous cast product.35. A method according to claim 18, wherein said preparing comprisescontinuous or semi-continuous casting of the core and the interlayer.36. A metallurgical product according to claim 2, wherein said interfacebetween said core and said interlayer is free of oxides andoxide-compounds.
 37. A metallurgical product according to claim 17,wherein said interface between said core and said interlayer is free ofoxides and oxide-compounds.
 38. A metallurgical product according toclaim 1, wherein said interlayer comprises an anodic interlayer.
 39. Ametallurgical product according to claim 17, wherein said interlayercomprises an anodic interlayer.
 40. A method according to claim 18,wherein the interlayer comprises an anodic interlayer.
 41. A methodaccording to claim 18, wherein said bonding zone forms by mutual mixingof the respective compositions of the core and interlayer at theirmutual interface in the molten stat prior to their solidification.